As Mary Ann was only about 16/17 years old and William, 19/20 years old, we have to assume that either her mother or William's parents did not give parental consent for the marriage to take place. At this time, you had to be 21 before you could marry without parental consent. Presumably, this was why the couple eloped to Gretna Green as there, they could be married on the spot with the only requirements being having 2 witnesses and giving an assurance that they were free to marry.